Solar heating panels used in water heating systems are generally mounted on a roof top or other exposed position and tilted at a suitable angle to intercept maximum solar energy at the geographic location. Desirably, the panel has a large surface to liquid volume ratio so that solar energy absorbed by the panel rapidly heats a thin layer of liquid (primarily water) in the panel. In a passive system, this creates a sufficient difference in liquid density at the top and bottom of the tilted panel to create a pumping action on the liquid to circulate it back to a heat storage tank. If the liquid is water, hot water may be withdrawn directly from the storage tank. More frequently, the solar heated liquid heats the domestic water supply through a heat exchanger in the storage tank.
Pumping liquid through the panel stops when there is not enough solar energy. In cold weather, particularly at night, the liquid freezes in the panel. Such freezing blocks circulation until solar energy (or warm weather) thaws the liquid. Expansion of internal fluid during freezing imposes substantial strains in the panel. Such strains are, of course, repetitive for each freeze and thaw cycle. Where a panel is formed of parallel tubes the strains usually are concentrated at the interconnected headers. Further, to accommodate such strains, the surface area to liquid volume ratio is generally reduced which in turn reduces the overall efficiency of the solar heating system. Additionally, the cost both in labor and parts to assemble a solar panel from a plurality of parallel tubes and headers substantially reduces the economic advantages of solar water heating systems over conventional fossil fuel systems.
It has been proposed heretofore to construct such solar liquid heating panels of sheets of metal, rubber or thermoset plastic materials bonded together at discrete locations over the surface of the sheets. Such panels, in general, have not been constructed to withstand adequately repetitive freeze and thaw cycles. I have found that such failures involve breaking of the bonds between the sheets so that the panel loses dimensional stability and at the same time uniformity in thickness of the liquid layer to be heated. Liquid flow then primarily passes through any resulting "bulges" since the enlarged areas are less resistant to flow. Such channeled flow reduces the heat exchange efficiency of the panel. Where the bonds are strong enough to resist such stresses, either the sheet must be increased in thickness, thereby adding to cost, or repeated stresses may crack the material with consequent loss of liquid which requires repair or replacement of the panel.
As disclosed in U.S. patent application Ser. No. 258,304, filed June 23, 1981 by H. W. Anderson and M. E. Negly, and U.S. patent application Ser. No. 258,519, filed June 22, 1981 by B. S. Buckley, both assigned to the assignee of the present invention, a suitable panel may be formed of two or more sheets of rubber, or other elastomeric material, bonded together about their edges and at uniformly spaced apart discrete areas. The elasticity of the material permits the stresses induced by freezing to be transferred to the material which is free to expand between the bonded areas. However, to provide such dimensional stability to the panel it must be supported on a bed over substantially its entire area so that it may be properly tilted to absorb maximum solar energy. In passive solar systems the storage tank and solar panel are usually mounted together, either tank-over-panel or back-to-back. Such a support bed is available at little extra cost in a back-to-back system, but may be at substantial added cost in a tank-over-panel arrangement. Further in some environments, the rubber may be subject to chemical attack over an extended period, such as ten to twenty years.